Enhancing WiFi Connectivity for Moving Vehicles

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Aruna Balasubramanian, Ratul Mahajan
Arun Venkataramani,  Brian N Levine, John Zahorjan
Interactive WiFi Connectivity
from Moving Vehicles
University of Massachusetts Amherst
Microsoft Research
University of Washington
Motivation
Increasing demand for network
access from vehicles
E.g., VoIP, Web, email
Cellular access expensive
Ubiquity of WiFi
Cheaper, higher peak throughput
compared to cellular
2
Our work
 
Internet
 
Given enough coverage, can WiFi technology be used to
access mainstream applications from vehicles?
3
 
Existing work shows
the feasibility of WiFi
access at vehicular
speeds
 focus on non-interactive
applications. e.g., road
monitoring
Talk outline
 
Can popular applications be supported using
vehicular WiFi today?
Performance is poor due to frequent disruptions
 
How can we improve application performance?
ViFi
, a new handoff protocol that significantly reduces
disruptions
 
Does ViFi really improve application performance?
VoIP, short TCP transfers
4
VanLAN: Our Vehicular Testbed
Uses MS campus vans
Base stations(BSes) are
deployed on roadside
buildings
Currently 2 vans, 11
BSes
5
Measurement study
 
 
Study 
application performance in vehicular WiFi setting
Focus on basic connectivity
 
Study performance of different handoff policies
 
Trace-driven analysis
Nodes send periodic packets and log receptions
 
 
 
 
6
Handoff policies studied
 
 
 
 
 
Practical hard handoff
Associate with one BS
Current 802.11
 
Ideal hard handoff
Use future knowledge
Impractical
7
Handoff policies studied
 
 
 
Practical hard handoff
Associate with one BS
Current 802.11
Ideal hard handoff
Use future knowledge
Impractical
Ideal soft handoff
Use all BSes in range
Performance upper
bound
8
Comparison of handoff policies
 
Practical hard handoff
 
Ideal hard handoff
 
Ideal soft handoff
 
Disruption
Summary
Performance of 
interactive applications 
poor when
using existing handoff policies
Soft handoff policy can decrease disruptions and
improve performance of interactive applications
9
Talk outline
Can popular applications be accessed using
vehicular WiFi?
How can we improve application performance?
ViFi, a practical diversity-based handoff protocol
Does ViFi really improve application performance?
VoIP, short TCP transfers
10
Designing a practical soft handoff policy
 Goal: Leverage multiple BSes in range
 
Not straightforward
Internet
 
11
Constraints in Vehicular WiFi
1. Inter-BS backplane often bandwidth-constrained
2. Interactive applications require timely delivery
3. Fine-grained scheduling of packets difficult
Why are existing solutions inadequate?
 
 
 
Opportunistic protocols for WiFi mesh (ExOR, MORE)
Uses batching: Not suitable for interactive applications
 
Path diversity protocols for enterprise WLANs (Divert
)
Assumes BSes are connected through a high speed back plane
 
Soft handoff protocols for cellular (CDMA-based)
Packet scheduling at fine time scales
Signals can be combined
 
 
 
 
 
 
12
ViFi protocol set up
 
Internet
 
 
Vehicle chooses 
anchor
 BS
Anchor responsible for vehicle’s
packets
 
Vehicle chooses a set of BSes in
range to be 
auxiliaries
e.g., B, C and D can be chosen as
auxiliaries
ViFi leverages packets overheard by
the auxiliary
 
 
 
 
 
 
13
ViFi protocol
 
 
(1)
Source transmits a packet
 
(2)
If destination receives, it
transmits an ack
 
(3)
If auxiliary overhears packet but
not ack, it 
probabilistically
 relays
to destination
 
(4)
If destination received relay, it
transmits an ack
 
(5)
If no ack within retransmission
interval, source retransmits
 
 
 
 
 
 
 
 
 
 
 
 
 
A
B
D
C
 
A
B
D
C
 
 
Downstream: Anchor to vehicle
 
Upstream: Vehicle to anchor
14
 
Dest
 
Source
 
Dest
 
Source
Why relaying is effective?
 
Losses are bursty
Independence
Losses from different senders independent
Losses at different receivers independent
 
 
 
 
 
 
 
 
 
 
A
B
D
C
Upstream
 
15
A
B
D
C
Downstream
 
Guidelines for probability computation
1. Make a collective relaying decision and limit the total
number of relays
2. Give preference to auxiliary with good connectivity
with destination
 
 
 
 
 
 
 
 
How to make a collective decision without per-packet
coordination overhead?
16
Determine the relaying probability
 
Goal: Compute relaying probability 
R
B 
 
of auxiliary B
 
Step 1: The probability that auxiliary B is 
considering relaying
C
B
 = P(B heard the packet) . P(B did not hear ack)
 
Step 2: The expected number of relays by B is
 
E(B) = 
C
B
 
¢
 
R
B
 
Step 3: Formulate ViFi probability equation, 
 E(x) = 1, x 
2
 auxiliary
 
to solve uniquely, set 
R
B
 proportional to  P(destination hears B)
 
Step 4: B estimates P(auxiliary considering relaying) and
P(destination heard auxiliary) for each auxiliary
 
 
 
 
 
 
 
 
 
 
 
 
17
ViFi: Practical soft handoff protocol uses probabilistic
relaying for coordination without per-packet
coordination cost
ViFi Implementation
Implemented ViFi in windows operating system
Use broadcast transmission at the MAC layer
No rate adaptation
Deployed ViFi on VanLAN BSes and vehicles
 
 
 
 
 
 
 
 
18
Talk outline
Can popular applications be accessed using
vehicular WiFi?
Due to frequent disruptions, performance is poor
How can we improve application performance?
ViFi, a practical diversity-based soft handoff protocol
Does ViFi really improve application performance?
19
Evaluation
 
Evaluation based on VanLAN
deployment
ViFi reduces disruptions
ViFi improves application
performance
ViFi’s probabilistic relaying is
efficient
 
Also in the paper: Trace-driven
evaluation on DieselNet testbed
at UMass, Amherst
Results qualitatively consistent
20
ViFi reduces disruptions in our
deployment
Practical hard handoff
 
ViFi
21
ViFi improves VoIP performance
 
> 100%
 
Disruption = When mean opinion score (mos) is lower than a threshold
 
Length of voice call before disruption
Use G.729 codec
22
 
seconds
 
ViFi
 
Practical hard
handoff
ViFi improves performance of short
TCP transfers
 
Median transfer time (sec)
 
> 50%
 
> 100%
 
Number of transfers before disruption
Workload: repeatedly download/upload 10KB files
 
Disruption = lack of progress for 10 seconds
23
 
ViFi
 
Practical
hard
handoff
ViFi uses medium efficiently
 
efficiency
Efficiency:
 
Number of unique packets delivered/ Number of packets sent
24
 
ViFi
 
Practical hard
handoff
Conclusions
Our work improves performance of interactive
applications for vehicular WiFi networks
Interactive applications perform poorly in vehicular
settings due to frequent disruptions
ViFi, a diversity-based handoff protocol significantly
reduces disruptions
Experiments on VanLAN shows that ViFi significantly
improves performance of VoIP and short TCP transfers
http://research.microsoft.com/netres/Projects/vanLAN/
25
Practical soft handoff
Internet
 
 AllBS itself is not  a practical protocol
 
 
 How does diversity even work in the downstream?
26
Distributed computation
 
 
 
 
 
 
 
 
 
 
B needs to compute for each auxiliary
(1) contending probabilities (2) P(V will hear from the auxiliary)
 
Can be computed using loss rates between the auxiliary and its neighbors
 
V
27
Distributed probability computation
 
 
 
 
 
 
 
 
 
 
V
 
B need to know for C and D
1.
Probability that C and D are
making a relaying decision
2.
Probability that vehicle will
hear from C and D
 
Both can be estimated using loss
probabilities between C and D
and their neighbors
 
Nodes exchange 2-hop loss rates
using beacons
 
B computes relaying probabilities
for B, C and D
C
28
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This research explores the feasibility of using WiFi technology to enable network access from moving vehicles, focusing on improving performance for mainstream applications. The study includes a measurement analysis in a vehicular WiFi setting, comparing different handoff policies to enhance application performance. The findings emphasize a new handoff protocol, ViFi, as a promising solution to reduce disruptions and improve connectivity. The VanLAN vehicular testbed is utilized for practical experimentation and evaluation.


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  1. Interactive WiFi Connectivity from Moving Vehicles Aruna Balasubramanian, Ratul Mahajan Arun Venkataramani, Brian N Levine, John Zahorjan University of Massachusetts Amherst Microsoft Research University of Washington

  2. Motivation Increasing demand for network access from vehicles E.g., VoIP, Web, email Cellular access expensive Ubiquity of WiFi Cheaper, higher peak throughput compared to cellular 2

  3. Our work Given enough coverage, can WiFi technology be used to access mainstream applications from vehicles? Existing work shows the feasibility of WiFi access at vehicular speeds focus on non-interactive applications. e.g., road monitoring Internet 3

  4. Talk outline Can popular applications be supported using vehicular WiFi today? Performance is poor due to frequent disruptions How can we improve application performance? ViFi, a new handoff protocol that significantly reduces disruptions Does ViFi really improve application performance? VoIP, short TCP transfers 4

  5. VanLAN: Our Vehicular Testbed Uses MS campus vans Base stations(BSes) are deployed on roadside buildings Currently 2 vans, 11 BSes 5

  6. Measurement study Study application performance in vehicular WiFi setting Focus on basic connectivity Study performance of different handoff policies Trace-driven analysis Nodes send periodic packets and log receptions 6

  7. Handoff policies studied Practical hard handoff Associate with one BS Current 802.11 Ideal hard handoff Use future knowledge Impractical 7

  8. Handoff policies studied Practical hard handoff Associate with one BS Current 802.11 Ideal hard handoff Use future knowledge Impractical Ideal soft handoff Use all BSes in range Performance upper bound 8

  9. Comparison of handoff policies Disruption Summary Performance of interactive applications poor when using existing handoff policies Soft handoff policy can decrease disruptions and improve performance of interactive applications Practical hard handoff Ideal hard handoff Ideal soft handoff 9

  10. Talk outline Can popular applications be accessed using vehicular WiFi? How can we improve application performance? ViFi, a practical diversity-based handoff protocol Does ViFi really improve application performance? VoIP, short TCP transfers 10

  11. Designing a practical soft handoff policy Goal: Leverage multiple BSes in range Not straightforward Constraints in Vehicular WiFi 1. Inter-BS backplane often bandwidth-constrained 2. Interactive applications require timely delivery 3. Fine-grained scheduling of packets difficult Internet 11

  12. Why are existing solutions inadequate? Opportunistic protocols for WiFi mesh (ExOR, MORE) Uses batching: Not suitable for interactive applications Path diversity protocols for enterprise WLANs (Divert) Assumes BSes are connected through a high speed back plane Soft handoff protocols for cellular (CDMA-based) Packet scheduling at fine time scales Signals can be combined 12

  13. ViFi protocol set up Vehicle chooses anchor BS Anchor responsible for vehicle s packets C A B Vehicle chooses a set of BSes in range to be auxiliaries e.g., B, C and D can be chosen as auxiliaries ViFi leverages packets overheard by the auxiliary D Internet 13

  14. ViFi protocol C Source (1) Source transmits a packet B A (2) If destination receives, it transmits an ack Dest D (3) If auxiliary overhears packet but not ack, it probabilistically relays to destination Downstream: Anchor to vehicle C Dest (4) If destination received relay, it transmits an ack A B Source D (5) If no ack within retransmission interval, source retransmits Upstream: Vehicle to anchor 14

  15. Why relaying is effective? Losses are bursty Independence Losses from different senders independent Losses at different receivers independent C C A B A B D D Downstream Upstream 15

  16. Guidelines for probability computation 1. Make a collective relaying decision and limit the total number of relays 2. Give preference to auxiliary with good connectivity with destination How to make a collective decision without per-packet coordination overhead? 16

  17. Determine the relaying probability Goal: Compute relaying probability RB of auxiliary B Step 1: The probability that auxiliary B is considering relaying CB = P(B heard the packet) . P(B did not hear ack) Step 2: The expected number of relays by B is E(B) = CB RB ViFi: Practical soft handoff protocol uses probabilistic relaying for coordination without per-packet coordination cost Step 3: Formulate ViFi probability equation, E(x) = 1, x 2 auxiliary to solve uniquely, set RB proportional to P(destination hears B) Step 4: B estimates P(auxiliary considering relaying) and P(destination heard auxiliary) for each auxiliary 17

  18. ViFi Implementation Implemented ViFi in windows operating system Use broadcast transmission at the MAC layer No rate adaptation Deployed ViFi on VanLAN BSes and vehicles 18

  19. Talk outline Can popular applications be accessed using vehicular WiFi? Due to frequent disruptions, performance is poor How can we improve application performance? ViFi, a practical diversity-based soft handoff protocol Does ViFi really improve application performance? 19

  20. Evaluation Evaluation based on VanLAN deployment ViFi reduces disruptions ViFi improves application performance ViFi s probabilistic relaying is efficient Also in the paper: Trace-driven evaluation on DieselNet testbed at UMass, Amherst Results qualitatively consistent 20

  21. ViFi reduces disruptions in our deployment ViFi Practical hard handoff 21

  22. ViFi improves VoIP performance Use G.729 codec 100 80 60 > 100% seconds ViFi Practical hard handoff 40 20 0 0.5 Length of voice call before disruption Disruption = When mean opinion score (mos) is lower than a threshold 22

  23. ViFi improves performance of short TCP transfers Workload: repeatedly download/upload 10KB files 100 1 80 0.8 > 50% 0.6 > 100% 60 ViFi 0.4 40 Practical hard handoff 0.2 20 0 0 Number of transfers before disruption Disruption = lack of progress for 10 seconds Median transfer time (sec) 0.5 0.5 23

  24. ViFi uses medium efficiently Efficiency: Number of unique packets delivered/ Number of packets sent 1 0.8 efficiency 0.6 ViFi 0.4 Practical hard handoff 0.2 0 0.5 24

  25. Conclusions Our work improves performance of interactive applications for vehicular WiFi networks Interactive applications perform poorly in vehicular settings due to frequent disruptions ViFi, a diversity-based handoff protocol significantly reduces disruptions Experiments on VanLAN shows that ViFi significantly improves performance of VoIP and short TCP transfers http://research.microsoft.com/netres/Projects/vanLAN/ 25

  26. Practical soft handoff AllBS itself is not a practical protocol How does diversity even work in the downstream? Internet 26

  27. Distributed computation B needs to compute for each auxiliary (1) contending probabilities (2) P(V will hear from the auxiliary) Can be computed using loss rates between the auxiliary and its neighbors Compute loss rates using beacon receptions C V Embeds its loss rates in beacons A B Learns loss rates of auxiliaries from their beacons D Estimates relaying probability of B, C and D 27

  28. Distributed probability computation Compute consistent relaying probabilities B need to know for C and D 1. Probability that C and D are making a relaying decision 2. Probability that vehicle will hear from C and D C V A B Both can be estimated using loss probabilities between C and D and their neighbors D Nodes exchange 2-hop loss rates using beacons Compute consistent relaying probabilities B computes relaying probabilities for B, C and D 28

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